With the Double Floor Lashing, the floor poles (floor spars, decking poles) are attached to each platform support (stringer) on both sides. This means, when pulled tight, this lashing makes the floor or platform more secure.
Start by tying a clove hitch around the platform support and wrap the tail around the the long part of the rope. Next, make a bend in the running end and pass it over the first floor pole on the inside of the platform support. Now, grab this bite from below and pass it underneath the platform support and loop it over the first floor pole on the outside of the platform support. Tighten both loops around the first floor pole by pulling the running end, which is extending out from the top of the platform floor between the first and second floor poles.
This process is repeated for each floor pole until you reach the other end. When you reach the other end, tie off the rope to the platform support with a series of tight half hitches.
While the ends of the floor poles are being lashed to one platform support, the other ends can simultaneously be lashed to the other platform support.